Magnetic recording media generally include a binder dispersion layer overlying a substrate. The binder dispersion layer may comprise a binder composition and a pigment, wherein the pigment is dispersed within the binder composition. Typically, the pigment is a magnetizable pigment comprising small, magnetizable particles. In some instances, the medium may be in the form of a composite having both back-coat and front-coat binder dispersion layers, although the pigment in the back-coat may or may not be a magnetizable pigment.
It has become desirable to have as high a loading of magnetizable pigment in the magnetic recording media as is reasonably possible. It is often preferred to have a binder dispersion comprising from about 70% to 80% by weight magnetizable pigment relative to the binder with as many magnetizable particles per unit area or unit volume as possible. It is also preferred to have a binder dispersion in which the magnetizable pigment comprises a plurality of small particles having a relatively high specific surface area. Higher pigment loading has the potential to provide high density magnetic recording media capable of storing more information.
Problems, however, remain in the art concerning magnetic recording media having a relatively high loading of magnetizable pigment. To begin with, magnetizable pigments tend to agglomerate, and they are difficult to properly and fully disperse within the binder. Wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loading, i.e., the use of greater amounts by weight and number of magnetizable particles, greater amounts of such dispersants are required, which is not always desirable
There are a number of reasons for using as little dispersant as possible. When used in large amounts, dispersants can reduce the durability of the resulting magnetic recording medium. Excess dispersant may also have a tendency to bloom from a cured binder dispersion over time, leading to contamination of a recording head or the like, or causing a change in the physical or chemical characteristics of the media. Larger amounts of dispersants may result in lower modulus of the magnetic coating. If the modulus is too low, and if a back-coat having a relatively rough surface is required, e.g., to obtain better handling performance of a tape at fast speeds, the rough back-coat may emboss the relatively smooth magnetic front-coat when the tape is wound upon itself. Such embossing will degrade the output of the tape due to spacing loss between the front-coat and a recording head. Costs, too, can be reduced by using less dispersant. Additionally, binder dispersions can be more readily and reproducibly prepared when less dispersant is used.
Another problem in the art is that the viscosity of a binder dispersion generally increases with higher loading of magnetizable pigment. If the dispersion is too viscous, it can be difficult to apply to the substrate, and good magnetic orientation of the pigment, i.e., a squareness ratio of 0.75 or more, can be hard to obtain. The squareness ratio (Br/Bm), which is the ratio of the remnant saturation induction, or residual magnetization (Br), to the saturation induction, or saturation magnetization (Bm), refers to the effectiveness of the orientation of the magnetic particles. For randomly-oriented particles, the squareness ratio is 0.5 and for ideally and perfectly oriented particles, the ratio is equal to 1.0. Values for the squareness ratio, of media exhibiting good performance, normally fall around 0.75 to 0.85, with higher values being significantly better. The difference between a 0.75 and a 0.85 squareness ratio typically represents about a 1 decibel improvement in electromagnetic characteristics, which is manifested by an approximate 10% improvement in electromagnetic properties such as signal output and/or signal-to-noise ratio. In general, an increase in the squareness ratio is reflected by an improvement in orientation characteristics and electromagnetic properties and an increase from 0.75 to 0.80 (i.e. an increase of 0.05) results in a significant advantage.
To help alleviate these problems with high pigment loading, binder compositions having internal dispersants have been developed. Such compositions comprise polymers with functional moieties pendant from the polymer backbone that help disperse the magnetizable pigment. As a result of using these compositions, less dispersant is needed for dispersion of magnetizable pigment in the binder. Unfortunately, the known compositions having internal dispersants have been ineffective, or inefficient, for obtaining magnetic recording media having a relatively high pigment loading and/or a relatively high magnetic orientation, i.e., squareness ratio.
One class of such compositions comprises sulfonated polyurethanes. See, for example, U.S. Pat. No. 4,152,485, incorporated herein by reference. Although the sulfonated polyurethanes exhibit low viscosity for providing good, initial magnetic orientation, they may not cure sufficiently to maintain such orientation over long periods and may have inferior durability. Generally, it is the curing of the binder that retains the magnetizable pigment in an oriented manner.
Another class of binder compositions having internal dispersants comprises hydroxy-functional, sulfonated polyurethanes. See, for example, Japanese Kokai 61-198417. The hydroxy-functional, sulfonated polyurethanes have excellent curing properties as a result of the cross-linking capability provided by the hydroxy functionality. Unfortunately, however, the known hydroxy-functional, sulfonated polyurethanes exhibit such high dispersion viscosity that good initial magnetic orientation is difficult, or impossible, to obtain.
It has also become desirable to increase the durability, running properties, and reliability of the magnetic recording media. To accomplish this, attempts have been made to add a hard material to the pigment-containing binder. One such hard material is a vinyl chloride copolymer containing internal dispersants, such as vinyl chloride copolymer having pendant carboxyl groups or pendant sulfonate groups. Such materials are disclosed, for example, in Japanese Kokai 61-026932 and U.S. Pat. No. 4,731,292. However, when binder compositions are prepared using such vinyl chloride copolymers together with conventional sulfonated polyurethanes, or sulfonated, hydroxy-functional polyurethanes, such binder compositions exhibit relatively high dispersion viscosity, and the resulting magnetic recording media would have a low squareness ratio. Thus, use of the vinyl chloride copolymers having pendant carboxyl groups or pendant sulfonate groups has not provided high quality magnetic recording media.
It has also become desirable to provide magnetic recording media having high "green strength", i.e., high glass transition temperature (Tg) and tough abrasion resistance in the uncured state, and high final coating modulus. High green strength and high final coating modulus are desirable and necessary where stiffer tapes are needed for better handling in the recording and playback machine, or for improved resistance to damage during manufacture or processing. For example, such properties are needed for magnetic recording tape in which a rough back-coat must be used for proper tape transport properties.